Apparatus and method for non-invasively measuring cardiac output

ABSTRACT

Apparatus and methods for non-invasively determining cardiac output using partial re-breathing techniques are disclosed in which the apparatus is constructed with an instantaneously adjustable deadspace for accommodating differences in breathing capacities of various patients. The apparatus is constructed of inexpensive elements, including a single two-way valve which renders the apparatus very simple to use and inexpensive so that the unit may be readily disposable. The method of the invention provides a novel means of estimating cardiac output based on alveolar CO 2  values rather than end-tidal CO 2  values as previously practiced. A program for calculating cardiac output is also disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 10/657,909,filed Sep. 8, 2003, pending, which is a divisional of application Ser.No. 09/767,363, filed Jan. 23, 2001, now U.S. Pat. No. 6,648,831, issuedNov. 18, 2003, which is a continuation of application Ser. No.08/770,138, filed Dec. 19, 1996, now U.S. Pat. No. 6,306,098, issuedDec. 23, 2001.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to non-invasive means of determining cardiacoutput in patients, and specifically relates to partial re-breathingsystems and methods for determining cardiac output in patients.

2. Background of Related Art

It is important in many medical procedures to determine or monitor thecardiac output of a patient. Techniques are known and used in the artwhich employ the use of catheters inserted at certain arterial points(e.g., femoral artery, jugular vein, etc.) to monitor blood temperatureand pressure in order to determine cardiac output of the patient.Although such techniques can produce a reasonably accurate result, theinvasive nature of the procedure has high potential for morbidity andmortality consequences.

Adolph Fick's measurement of cardiac output, first proposed in 1870, hasserved as the standard by which all other means of determining cardiacoutput have been evaluated since that date. Fick's well-known equation,written for CO₂, is:$Q = \frac{V_{{CO}_{2}}}{\left( {C_{v_{{CO}_{2}}} - C_{a_{{CO}_{2}}}} \right)}$where Q is cardiac output, Vco₂ is the amount of CO₂ excreted by thelungs and C_(a) _(CO) ₂ and C_(v) _(CO) ₂ are the arterial and venousCO₂ concentrations, respectively. Notably, the Fick Equation presumes aninvasive method (i.e., catheterization) of calculating cardiac outputbecause the arterial and mixed venous blood must be sampled in order todetermine arterial and venous CO₂ concentrations.

It has previously been shown, however, that non-invasive means may beused for determining cardiac output while still using principlesembodied in the Fick Equation. That is, expired CO₂ (“pCO₂”) levels canbe monitored to estimate arterial CO₂ concentrations and a varied formof the Fick Equation can be applied to evaluate observed changes in pCO₂to estimate cardiac output. One use of the Fick Equation to determinecardiac output in non-invasive procedures requires the comparison of a“standard” ventilation event to a sudden change in ventilation whichcauses a change in expired CO₂ values and a change in excreted volume ofCO₂. The commonly practiced means of providing a sudden change ineffective ventilation is to cause the ventilated patient to re-breathe aspecified amount of previously exhaled air. This technique has commonlybeen called “re-breathing.”

Prior methods of re-breathing have used the partial pressure ofend-tidal CO₂ to approximate arterial CO₂ while the lungs act as atonometer to measure venous CO₂. That method of re-breathing has notproven to be a satisfactory means of measuring cardiac output becausethe patient is required to breathe directly into and from a closedvolume in order to produce the necessary effect. However, it is usuallyimpossible for sedated or unconscious patients to actively participatein inhaling and exhaling into a bag. The work of some researchersdemonstrated that the Fick Equation could be further modified toeliminate the need to directly calculate venous PCO ₂ (PVCO ₂) byassuming that the PVCO ₂ does not change within the time period of theperturbation, an assumption that could be made by employing the partialre-breathing method. (See, Capek et al., “Noninvasive Measurement ofCardiac Output Using Partial CO₂ Rebreathing,” IEEE Transactions OnBiomedical Engineering, Vol. 35, No. 9, September 1988, pp. 653-661.)

Known partial re-breathing methods are advantageous over invasivemeasuring techniques because they 1) are non-invasive, 2) use theaccepted Fick principle of calculation, 3) are easily automated, 4)require no patient cooperation and 5) allow cardiac output to becalculated from commonly monitored clinical signals. However, knownpartial re-breathing methods have significant disadvantages as well.Specifically, known methods 1) are less accurate with non-intubated orspontaneously breathing patients, 2) only allow intermittentmeasurements (usually about every four minutes), 3) result in anobserved slight, but generally clinically insignificant, increase inarterial C0₂ levels, and 4) do not permit measurement of shunted bloodflow (that is, blood which does not participate in gas exchange).Further, known apparatus used for partial re-breathing techniques are ofstandard construction and do not compensate for differences in patientsize or capacities. In addition, many devices employ expensive elements,such as three-way valves, which render the devices too expensive to beused as disposable units.

Thus, it would be advantageous to provide a means of measuring cardiacoutput using partial re-breathing techniques which 1) overcome thedisadvantages of prior systems, 2) provide better and more continuousmeasurement, and 3) require less expensive equipment, thereby making thedevice suitable for manufacturing as a single-use, or disposable,product. It would also be advantageous to provide partial re-breathingapparatus which is instantaneously adjustable to compensate for varioussizes and capacities of patients. Further, it would be advantageous toprovide new methods of estimating cardiac output based on alveolar CO₂output rather than end-tidal CO₂ as is currently used in the art.

SUMMARY OF THE INVENTION

In accordance with the present invention, apparatus and methods formeasuring cardiac output using a modified Fick Equation are providedwhere the amount of deadspace which is provided in the apparatus can beadjusted to increase or decrease the volume of exhalate to bere-breathed by the patient, thereby decreasing ventilation withoutchanging airway pressure. The apparatus and methods of the presentinvention also provide an adjustability factor which enables theapparatus to be adjusted to suit any size or capacity of patient. Theapparatus of the present invention also employs significantly lessexpensive elements of construction, thereby rendering the devicedisposable.

The apparatus and methods of the present invention apply a modified FickEquation to calculate changes in pCO₂ flow and concentration to evaluatecardiac output. The traditional Fick Equation, written for CO₂ is:$Q = \frac{V_{{CO}_{2}}}{\left( {C_{v_{{CO}_{2}}} - C_{a_{{CO}_{2}}}} \right)}$where Q is cardiac output (when calculated using re-breathing techniquesreferred to as pulmonary capillary blood flow or “PCBF”), VCO ₂ is theoutput of CO₂ from the lungs and C_(a) _(CO) ₂ and C_(v) _(CO) ₂ are thearterial and venous CO₂ concentrations, respectively. It has been shownin prior work of others that cardiac output can be estimated fromcalculating the change in pCO₂, as estimated by end-tidal CO₂ (“etCO₂”),as a result of a sudden change in ventilation. That can be done byapplying a differential form of the Fick Equation as follows:$Q = {\frac{V_{{CO}_{2_{1}}}}{\left( {C_{v1} - C_{a1}} \right)} = \frac{V_{{CO}_{2_{2}}}}{\left( {C_{v2} - C_{a2}} \right)}}$where C_(a) is arterial CO₂ concentration, C_(v) is venous CO₂concentration, and the subscripts 1 and 2 reference measured valuesbefore a change in ventilation and measured values during a change inventilation, respectively. The differential form of the Fick Equationcan, therefore, be rewritten as:$Q = \frac{V_{{CO}_{2_{1}}} - V_{{CO}_{2_{2}}}}{\left( {C_{v1} - C_{a1}} \right) - \left( {C_{v2} - C_{a2}} \right)}$or$Q = {\frac{\Delta\quad V_{{CO}_{2}}}{\Delta\quad C_{a_{{CO}_{2}}}} = \frac{\Delta\quad V_{{CO}_{2}}}{s\quad\Delta\quad{{et}{CO}}_{2}}}$where ΔV_(CO) ₂ is the change in CO₂ production in response to thechange in ventilation, ΔC_(a) _(CO) ₂ is the change in arterial CO₂concentration in response to the change in ventilation, ΔetCO₂ is thechange in end-tidal CO₂ concentration and s is the slope of the CO₂dissociation curve. The foregoing differential equation assumes thatthere is no appreciable change in venous CO₂ concentration during there-breathing episode, as demonstrated by Capek, et al., in theirprevious work. Also, a dissociation curve, well-known in the art, isused for determining C0₂ concentration based on partial pressuremeasurements.

In previous partial re-breathing methods, a deadspace, usuallycomprising an additional 50-250 ml capacity of air passage, was providedin the ventilation circuit to decrease the effective alveolarventilation. In the present invention, a ventilation apparatus isprovided with an adjustable deadspace to provide the necessary change inventilation for determining accurate changes in CO₂ production andend-tidal CO₂ commensurate with the requirements of differently sizedpatients. In one embodiment of the ventilation apparatus, selectivelyadjustable deadspace is provided through which the patient exhales andinhales. Thus, the adjustable deadspace of the apparatus permits easyadjustment of the deadspace to accommodate any size or capacity ofpatient, from a small to a large adult. As a result, the patient isprovided with a volume of re-breathable gas commensurate with thepatient's size which decreases effective ventilation without changingthe airway pressure. Because airway and intra-thoracic pressure are notaffected by the re-breathing method of the present invention, cardiacoutput is not significantly affected by re-breathing. In an alternativemethod, the deadspace may be effectively lessened by selectively leakingexhaled gas from the ventilation system to atmosphere or to a closedreceptacle means during inspiration.

The ventilation apparatus of the present invention includes a tubularportion, which is placed in contact with the patient, and an inhalationconduit and exhalation conduit. In a common configuration, theinhalation conduit and exhalation conduit may be interconnected betweena ventilator unit and the patient. Alternatively, however, a ventilatorunit (i.e., a source of deliverable gas mechanically operated to assistthe patient in breathing) need not be used with the ventilationapparatus and inhaled and exhaled breath is merely taken from or ventedto atmosphere. Other conventional equipment commonly used withventilator units or used in ventilation of a patient may be used withthe inventive ventilation apparatus, such as a breathing mask.

An electrical pneumotachometer for measuring flow of gas and acapnograph for measuring CO₂ concentrations are provided in proximity tothe tubular portion between the inhalation and exhalation portions ofthe ventilation apparatus and the patient's lungs. The pneumotachometerand capnograph serve as detection apparatus for detecting changes in gasconcentrations and flow and are in electrical communication with acomputer having software designed to store and evaluate the measurementstaken by the detection apparatus in real time. Other forms of detectionapparatus may be used. Adjustable deadspace means are provided inconnection with the exhalation portion of the ventilation apparatus, andmay interconnect with the inhalation portion of the ventilationapparatus. In one embodiment, the adjustable deadspace means may bemanually adjusted. Alternatively, electromechanical means may beinterconnected between the computer and the adjustable deadspace meansto provide automatic adjustment of the deadspace volume responsive tothe size or capacity of the patient and responsive to changes inventilation.

In an alternative embodiment, a tracheal gas insufflation apparatus isused to provide the change in ventilation necessary to calculatepulmonary CO₂ changes using the differential Fick Equation. Tracheal gasinsufflation (“TGI”) apparatus is commonly used to flush the deadspaceof the alveolar spaces of the lungs and to replace the deadspace withfresh gas infused through insufflation means. That is, fresh gas isintroduced to the central airway to improve alveolar ventilation and/orto minimize ventilatory pressure requirements. TGI apparatus isinterconnected to a ventilator system and includes a means ofintroducing fresh gas into the breathing tube as it enters the patient'slungs. The TGI apparatus may be used in the methods of the presentinvention to determine baseline measurements of VCO ₂ and etCO₂ duringTGI. When the TGI system is turned off, a deadspace is formed by thepatient's trachea and the endotracheal tube of the TGI apparatus whichallows measurement of a change in CO₂ to be evaluated in accordance withthe invention. Further, the catheter of the TGI apparatus may bevariably positioned within the trachea of the patient to further adjustthe deadspace volume.

The deadspace provided in the apparatus of the present invention causesa rapid drop in VCO ₂ which thereafter increases slightly and slowly asthe functional residual lung gas capacity equilibrates with the increasein alveolar CO₂ level. The change in etCO₂ rises more slowly after theaddition of deadspace, depending on alveolar deadspace and cardiacoutput, but then stabilizes to a new level. A “standard,” or baseline,breathing episode is conducted for a selected period of time immediatelypreceding the introduction of a deadspace (i.e., re-breathing) and VCO ₂and etCO₂ values are determined based on measurements made during the“standard” breathing event. Those values are substituted as the valuesV_(CO) ₂ ₁ and C_(a) _(CO) ₂ ₁ in the differential Fick Equation. VCO ₂and etCO₂ values are also determined from measurements takenapproximately thirty seconds following the introduction of a deadspaceduring partial re-breathing to provide the second values (subscript 2values) in the differential Fick Equation. The period of time duringwhich partial re-breathing occurs and during which normal breathingoccurs may be determined by the individual size and lung capacity of thepatient. Additionally, the period of time between a re-breathing episodeand a subsequent normal breathing episode may vary between patients,depending on a particular patient's size and breath capacity. Thus, athirty second time period for a breathing episode is only an averagetime and may be greater or lesser.

Cardiac output is determined, in the present invention, by estimatingalveolar CO₂ concentration rather than basing output on end-tidal CO₂concentration, as is practiced in the prior art. Partial pressure valuesthat are obtained from CO₂ measurements are converted to a value for gascontent in the blood using the dissociation equation known in the art.Thus, a more accurate cardiac output can be determined. In addition, theaccuracy of cardiac output is increased by correcting VCO ₂ values toaccount for flow of CO₂ into the functional residual capacity of thelungs, defined as the volume of gas left in the lungs at the end of anexpired breath. The determination of values based on experiential datais processed by the software program to determine cardiac output.

The ventilation apparatus of the present invention employs inexpensiveyet accurate monitoring systems as compared to the systems currentlyused in the art. The methods of the invention allow automaticadjustability of the apparatus for accommodating patients of differentsizes and provide consistent monitoring with modest recovery time.Further, the present apparatus and methods can be used equally withnon-responsive, intubated patients as well as non-intubated, responsivepatients.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

In the drawings, which illustrate what is currently considered to be thebest mode for carrying out the invention:

FIG. 1 is a schematic representation of conventional ventilation systemsused for assisting patient breathing;

FIG. 2 is a schematic representation of prior art re-breathing systems;

FIG. 3 is a schematic first embodiment of the ventilation apparatus ofthe present invention illustrating an adjustably expandable deadspace;

FIG. 4 is a schematic representation of an alternative embodiment of thepresent invention where the re-breathing circuit is constructed with aleak valve;

FIGS. 5(A)-(C) are schematic representations of another alternativeembodiment of the present invention where the inhalation portion andexhalation portion of the ventilation circuit are interconnected andhave a two-way closeable valve;

FIG. 6 is a schematic representation of an alternative embodimentsimilar to the embodiment shown in FIGS. 5(A)-(C), but where theinhalation and exhalation portions are adjustably expandable;

FIG. 7 is a schematic representation of another embodiment of theinvention where a series of valves is provided along the length of theinhalation and exhalation portions to provide a selectable volume ofdeadspace dependent upon the size and/or capacity of the patient;

FIGS. 8(A) and (B) are schematic representations of another embodimentof the invention where a leak valve is provided, both with a vent toatmosphere and to a parallel compliance chamber;

FIG. 9 is a schematic representation of a tracheal gas insufflationapparatus of the present invention which can be used to provide anecessary change in ventilation with a deadspace;

FIG. 10 is a schematic representation of human lungs illustrating theconcepts of parallel deadspace, alveolar deadspace and serial deadspacein the lungs of a patient;

FIG. 11 is a flow diagram briefly describing the calculations made inthe software program to calculate cardiac output from the measuredvalues of normal breathing and partial re-breathing; and

FIG. 12 is a schematic representation of an alternative embodiment ofthe invention shown in FIG. 7 which includes variably expandableinhalation and exhalation portions.

DETAILED DESCRIPTION OF THE INVENTION

For comparative purposes, FIG. 1 schematically illustrates aconventional ventilation system which is typically used with patientswho require assisted breathing during an illness, a surgical procedureor recovery from a surgical procedure. The conventional ventilatorsystem 10 includes a tubular portion 12 which is inserted into thetrachea by intubation procedures. The distal end 14 of the tubularportion 12 is fitted with a Y-piece 16 which interconnects aninspiratory hose 18 and an expiratory hose 20. Both the inspiratory hose18 and expiratory hose 20 are connected to a ventilator machine (notshown) which delivers air to the inspiratory hose 18. A one-way valve 22is positioned on the inspiratory hose 18 to prevent exhaled gas fromentering the inspiratory hose 18 beyond the valve 22. A similar one-wayvalve 24 on the expiratory hose 20 limits movement of inspiratory gasinto the expiratory hose 20. Exhaled air flows passively into theexpiratory hose 20.

In known re-breathing ventilation circuits 30, as shown in FIG. 2, thetubular portion 32 is inserted into the trachea of the patient byintubation procedures, and gas is provided to the patient from aventilator machine (not shown) via an inspiratory hose 34 which isinterconnected by a Y-piece 36 to an expiratory hose 38. An additionallength of hose 40 is provided between the tubular portion 32 and theY-piece 36 which acts as a deadspace for receiving exhaled gas. Athree-way valve 42, generally positioned between the Y-piece 36 and theopening to the additional length of hose 40, is constructed forintermittent actuation to selectively direct the flow of gas. That is,at one setting, the valve 42 allows inspiratory gas to enter the tubularportion 32 while preventing movement of the gas into the additionallength of hose 40. In a second setting, the valve 42 allows exhaled gasto enter into the expiratory hose 38 while preventing movement of gasinto the additional length of hose 40. In a third setting, the three-wayvalve 42 directs exhaled air to enter into the additional length of hose40 and causes the patient to re-breathe the exhaled air on the followingbreath to thereby cause a change in effective ventilation.

The change in VCO ₂ and end-tidal CO₂ caused by the change inventilation in the prior art system of FIG. 2 can then be used tocalculate cardiac output. Sensing and/or monitoring devices may beattached to the re-breathing ventilation circuit 30 between theadditional length of hose 40 and the tubular portion 32. The sensingand/or monitoring devices may include, for example, means for detectingCO₂ concentration 44 and means for detecting flow parameters 46 duringinhalation and exhalation. Those sensing and/or monitoring devices aretypically connected to data recording and display equipment (not shown).One problem encountered in use of the prior art system is that thedeadspace provided by the additional length of hose 40 is fixed and maynot be adjusted. As a result, the amount of deadspace provided in thecircuit for a small adult to effect re-breathing is the same amount ofdeadspace available for a large adult to effect re-breathing, and theresulting changes in CO₂ values for patients of different size, derivedfrom fixed-deadspace systems, can produce inadequate evaluation ofcardiac output. Further, the three-way valve 42 of the system isexpensive and significantly increases the cost of the ventilationdevice.

FIG. 3 illustrates the ventilation apparatus of the present inventionwhich provides an improvement over known ventilation devices used todetect or monitor cardiac output. The present ventilation apparatus 50comprises a tubular airway 52 which is placed in communication with thepatient's lungs. Although the present ventilation apparatus 50 may beplaced in communication with the trachea by intubation procedures as isdone in the prior art, the present ventilation apparatus 50 need not beinserted directly into the trachea of the patient. Alternatively, abreathing mask may be used for positioning over the patient's nose andmouth. Thus, the present invention may be used with unconscious oruncooperative patients needing ventilation assistance and may be usedwith equal efficacy with patients who are conscious. The ventilationapparatus 50 also includes an inspiratory hose 54 and an expiratory hose56 which may each be ventilated to atmosphere or connected to aventilator machine 55 (shown in phantom) which provides gas for deliveryto the patient through the inspiratory hose 54. The inspiratory hose 54and expiratory hose 56 may be joined together by a Y-piece 58.

The Y-piece 58 connects to an additional length of conduit or hose 60which provides a deadspace for receiving exhaled gas from the patient.However, the additional length of hose 60 is structured to beselectively expandable to readily enable the volume of deadspace to beadjusted commensurate with the size or lung capacity of the patient, orto other ventilation parameters, such as increased or decreased tidalvolume or modified respiration rate. As suggested by the schematicdrawing of FIG. 3, selective expansion of the deadspace may beaccomplished by structuring the additional length of hose 60 with anexpandable section 62 made of, for example, a piece of corrugated hosewhich can be lengthened or shortened by simply pulling or pushing theexpandable section 62 along its longitudinal axis 64. The corrugatedhose will retain the length at which it is positioned until adjustedagain. Other suitable means of providing adjustable expansion of thevolume of the deadspace are available, extending the length of the hose60 being but one approach. A three-way valve 68 may be connected to theadditional length of hose 60 to force inspiratory gas to enter thedeadspace 70 upon inhalation. The three-way valve 68 is also structuredto selectively prevent exhaled gas from entering the deadspace 70 duringnormal breathing or to direct exhaled gas into deadspace 70 duringre-breathing episodes so that the patient is forced to re-breatheexhaled gas from the deadspace 70.

A flow meter 72, or pneumotachometer, is attached to the ventilationapparatus 50 at a point between the tubular airway 52 and the additionallength of hose 60. The flow meter 72 detects gas flow through theventilation apparatus 50. A CO₂ sensor 74, or capnograph, is alsoconnected to the ventilation apparatus 50 between the tubular airway 52and the additional length of hose 60. The CO₂ sensor 74 detects changesin CO₂ resulting from a change in ventilation, the data from which isused to calculate cardiac output. The CO₂ sensor 74 may be an “onairway” sensor, a sampling sensor of the type which withdraws a sidestream sample of gas for testing, or any other suitable CO₂ sensor. Boththe flow meter 72 and CO₂ sensor 74 are connected to a computer 76 whichis programmed to store and analyze data from the flow meter 72 and CO₂sensor 74, and to calculate from the data the estimated cardiac outputof the patient.

As previously described herein, the differential Fick Equation requiresa change in pulmonary gas concentration and output to be induced in thepatient in order to estimate cardiac output. Re-breathing gas previouslyexhaled by the patient increases the amount of CO₂ breathed in by thepatient and enables the evaluation of increased CO₂ levels during achange in effective ventilation as compared to standard CO₂ levelsduring normal ventilation. The ventilation apparatus of the presentinvention provides the ability to selectively adjust the deadspacerequired in re-breathing to increase the amount of gas (CO₂) re-breathedby the patient from the previous exhalation. The ventilation apparatusof the present invention also allows the ventilation circuit to beadjusted automatically in accordance with the size or capacity of apatient, and in response to ventilation parameters. That is, if thedetected change in etCO₂ is less than 3 mm Hg, or the change in VCO ₂ isless than 0.2 times the VCO ₂, then the deadspace volume should beincreased by twenty percent.

In an alternative embodiment of the apparatus 50 of the invention, asshown in FIG. 4, the expense of using a three-way valve may beeliminated by structuring the additional length of hose 60 with aninexpensive two-way valve 78 and by positioning a flow restrictor 80between the inlet 82 and outlet 83 of the deadspace 70. Thus, when thetwo-way valve 78 is closed, gas to and from the ventilator machine 55will be directed through the flow restrictor 80 and to the patient.During a re-breathing episode, the two-way valve 78 is open so that theexhaled air encountering the flow restrictor 80 follows the course ofless resistance through the deadspace 70. Thus, the deadspace 70 may beadjusted at the expandable section 62 to provide the necessary deadspace70 for calculating changes in cardiac output.

In another alternative embodiment of the ventilation apparatus 50 of thepresent invention, as shown in FIGS. 5(A)-(C), a shunt line 84 ispositioned between the inspiratory hose 54 and the expiratory hose 56 toprovide selectively sized deadspace 70 in the circuit. The structure ofthe embodiment shown in FIGS. 5(A)-(C) causes the inspiratory hose 54and expiratory hose 56 to act as part of the deadspace 70, as well. Atwo-way shunt valve 86 positioned on the shunt line 84 selectivelydirects the flow of inspired and expired gas dependent upon whether thetwo-way shunt valve 86 is open or closed. Thus, when the ventilationapparatus 50 is configured for normal or baseline breathing, as depictedin FIG. 5(A), exhaled air (represented by the shaded area) will enterthe expiratory hose 56. During normal breathing, the two-way shunt valve86 is closed. When the ventilation apparatus 50 is configured for are-breathing episode, as depicted in FIG. 5(B), the two-way shunt valve86 is opened and exhaled gas may fill a portion of the inspiratory hose54, all of the expiratory hose 56 and the shunt line 84, all of whichserve as the deadspace 70.

The deadspace 70 in the embodiment shown in FIGS. 5(A)-(C) may berendered adjustably expandable, as shown in FIG. 6, by structuring theinspiratory hose 54 with an expandable section 90 positioned between theshunt line 84 and the Y-piece 58, and by structuring the expiratory hose56 with an expandable section 92 positioned between the shunt line 84and the Y-piece 58. Thus, the deadspace 70 can be selectively adjustedin accordance with the size or capacity of the patient, or responsive tooperating conditions, by increasing that portion of the inspiratory hose54 and expiratory hose 56 extending from the Y-piece 58. Any suitableadjustably expandable means may be used. As suggested by FIG. 6,however, the expandable section 90, 92 may be made from corrugatedplastic material, the length of which can be easily expanded orcontracted, and the plastic material will maintain its adjusted lengthuntil repositioned. The embodiment of FIG. 6 provides a particularlysimple and inexpensive construction rendering a particularly preferredembodiment because of its ease of use and disposability.

In yet another embodiment of the ventilation apparatus 50′ of thepresent invention, as shown in FIG. 7, the amount of available deadspace70 may be selectively adjusted by providing a plurality of shunt lines84, 94, 96 positioned between the inspiratory hose 54 and the expiratoryhose 56, with each shunt line 84, 94, 96 being structured with a two-wayshunt valve 86, 98, 100. In operation, the amount of deadspace 70required, as dictated by the size or capacity of the patient, may beselectively provided by using any suitable number of shunt lines 84, 94,96 to allow exhaled gas to move through the ventilation apparatus 50′.For example, given a patient of average size or lung capacity, it may beappropriate to use the first shunt line 84 and the second shunt line 94as potential deadspace 70. Thus, as the patient exhales in are-breathing episode, the two-way shunt valves 86, 98 associated withthe first shunt line 84 and second shunt line 94 may be opened, allowingexhaled and re-breathable gas to fill the expiratory hose 56, theinspiratory hose 54 between the second shunt line 94 and the Y-piece 58,the first shunt line 84 and the second shunt line 94. With a patient oflarger size or greater lung capacity, it may be necessary to use thethird shunt line 96 as well in providing sufficient deadspace 70 forre-breathing. Notably, each two-way shunt valve 86, 98, 100 may be inelectromechanical communication with the computer 76 (not shown in FIG.7) so that the computer may determine from the pneumotachometer, forexample, that additional deadspace 70 is required and cause the openingof one or more of the two-way shunt valves 86, 98, 100 to providesufficient additional deadspace 70. In an alternative embodiment, theventilation apparatus 50′ shown in FIG. 7 may be modified by theaddition of selectively expandable sections 90, 92 as shown in FIG. 12.

In the several alternative embodiments of the invention previouslyillustrated and described, the amount or volume of the deadspace hasbeen selectively adjustable by providing means for adjusting the volumeof the deadspace, such as by providing length-expanding means. It may beequally appropriate, however, to provide a change in ventilation, asrequired by the differential Fick Equation, by leaking some of theexhaled gas out of the system during the inspiration phase of a breath.Thus, as illustrated by FIG. 8(A), the ventilation apparatus 50 of thepresent invention may be structured with an evacuation line 106connected to the expiratory hose 56 of the ventilation apparatus 50. Theevacuation line 106 may be structured with gas releasing structure, suchas a simple valve 108 connected thereto which, when opened, allowsexhaled gas to move through the evacuation line 106. An orifice 110positioned at the end of the evacuation line 106 allows some of theexhaled gas to escape to the atmosphere.

When and how much exhaled gas should be leaked from the ventilationapparatus 50 during a re-breathing event may be determined by thecomputer (not shown in FIG. 8) in response to flow conditions, CO₂conditions and/or the size or lung capacity of the patient. The valve108, in electromechanical communication with the computer, may beselectively actuated according to ventilation or patient conditions.Where a patient is anesthetized or is otherwise exhaling gas which isundesirable for venting to the atmosphere, a compliant chamber 112, suchas an expandable bag shown in FIG. 8(B), may be attached to theevacuation line 106 to receive the exhaled gas leaked from theventilation circuit.

FIG. 9 schematically illustrates the use of a tracheal gas insufflation(TGI) apparatus 120 to provide the necessary deadspace in determiningcardiac output in patients. TGI apparatus is typically used to ventilatesick patients who require the injection of fresh gas into their centralairway for the improvement of alveolar ventilation. TGI apparatus can beconfigured to provide continuous or phasic (e.g., only duringinhalation) injections of gas. The TGI apparatus supplies gas, or anoxygen/gas mixture, to the lungs with every breath. As shown in FIG. 9,the TGI apparatus comprises an endotracheal tube 122 which is insertedinto the trachea 124 of the patient by intubation procedures. A catheter126 extends through the endotracheal tube 122 and into the patient'slungs, typically just above the carina. Gas or an oxygen/gas blend isprovided from a gas source 128 and is directed through gas tubing 130into the catheter 126. A flow meter 132 may assist in determining theoptimum amount of gas to be introduced into the lungs.

An adaptor fitting 134 may be used to connect a ventilation circuit 136of a type previously described to the TGI apparatus 120. That is, aventilation circuit 136 comprising a Y-piece 58 from which extends aninspiratory hose 54 and an expiratory hose 56 is structured with a flowmeter line 71 (not shown) attachable to a flow meter 72 and a CO₂ sensor74 for collecting data derived during a re-breathing event. In theillustrated TGI apparatus 120, the endotracheal tube 122 providesdeadspace required for re-breathing in addition to the ventilationcircuit 136 as previously described. To act as a deadspace, however, theTGI apparatus (i.e., the gas source 128 and flow meter 132) must beturned off, reduced or otherwise disabled. Exhaled air is therebyallowed to fill the endotracheal tube 122 and enter through the Y-piece58. The endotracheal tube 122 and ventilation circuit 136 serve asdeadspace when the TGI apparatus 120 is turned off. The volume ofdeadspace provided by the TGI apparatus configuration may be furtherincreased or decreased, as necessary, by varying the depth to which thecatheter 126 is positioned in the patient's trachea.

The computer to which the flow meter 72 and CO₂ sensor 74 are connectedis programmed to receive data collected by the flow meter 72 and CO₂sensor 74 and to analyze the data to calculate an estimated cardiacoutput. The parameters which are required by the software program toanalyze the data and to estimate cardiac output are described hereafter.

The calculation of cardiac output for a given patient is based on thecollection of data from the CO₂ sensor and flow meter attached to theventilation apparatus of the present invention. Raw flow and CO₂ signalsfrom the flow meter and CO₂ sensor 74 are filtered to remove artifactsand the flow signals, CO₂ signals and pressure signals are stored in abuffer in the software program. When the flow signal crosses aprescribed threshold (e.g., 15 liters/minute), the buffer is searched tofind the most recent zero-crossing. The zero-crossing is identified asthe start of a new breath. All data stored in the buffer since the lastzero-crossing and the crossing of the prescribed threshold (i.e., thenew zero-crossing) is established as one breathing cycle. For eachbreathing cycle, the parameters of the breathing phase are calculated asfollows:

-   -   1) etCO₂: The average concentration of CO₂ during the final 5%        of expiratory tidal volume is taken as end-tidal CO₂.    -   2) VCO ₂: The integral of flow (in milliliters) multiplied by        concentration of CO₂ over the entire breath is VCO ₂.    -   3) Inspired CO₂: This is the concentration of inspired CO₂. It        is the integral of CO₂ concentration times the volume (in        milliliters) of air flow during inspiration (i.e., negative        flow).    -   4) Airway deadspace: Determined as the expired volume (in        milliliters) at which CO₂ concentration crosses a selected        threshold set at, for example, 0.5 times etCO₂.

The initial values of VCO ₂ and etCO₂ are filtered employing athree-point median filter. The etCO₂ and VCO ₂ signals are straight-lineinterpolated and re-sampled at 0.5 Hz.

A correction is made in the VCO ₂ value to account for alveolardeadspace. That is, the correction in VCO ₂ corrects for the flow of CO₂into lung stores such as the functional residual capacity (FRC) in thelungs, or, in other words, the volume of gas left in the lungs at theend of a breath. Alveolar deadspace is demonstrated more clearly in FIG.10, which schematically illustrates the lungs 150 of a patient. Thelungs 150 generally comprise the trachea 152, bronchi 154 and alveoli156. The trachea 152 and bronchi 154 generally comprise what is known asthe anatomic or serial deadspace, which exists in the region indicatedbetween arrows A and B. In the lungs 150, there are alveoli 156 whichare perfused with blood (i.e., in contact with blood flow to provideoxygenation to the blood) and alveoli which are not perfused, thoughboth perfused and unperfused alveoli 156 may be ventilated.

Perfused alveoli 160 and unperfused alveoli 162 are illustrated in FIG.10. The perfused alveoli 160 are contacted with blood flowing throughminute capillaries 164 surrounding the alveoli 160, 162 the venous blood166 flowing toward the alveoli 160 and the arterial blood 168 flowingaway from the alveoli 160 in the direction of arrow 170. In the alveoli160, 162 a volume of gas known as the functional residual capacity (FRC)176 remains following exhalation. A portion 172 of the alveoli 160, 162which is evacuated upon exhalation (i.e., is ventilated) isrepresentational of alveolar CO₂ (PACO₂). In unperfused alveoli 162, theFRC 176 contains gas which is not evacuated during a breath, and theventilated portion 178 of the alveoli 162 forms a space containing gasor CO₂ which is ventilated but not perfused. It is the ventilatedportion 178 existing in the unperfused alveoli 162 which comprisesparallel deadspace (PDS), so called because it is ventilated in parallelwith the perfused alveoli.

In the present invention, the software program compensates, or accounts,for the functional residual capacity of the patient's lungs and thealveolar deadspace which exists. The correction is equal to the FRCtimes the change in end-tidal concentration orVCO ₂=VCO ₂+FRC×ΔetCO₂/Pbar,where “Pbar” is barometric pressure. FRC is estimated as a function ofbody weight as estimated by the deadspace volume using the equationFRC=FRC-factor x airway deadspace+an offset value,where the FRC-factor is a value experimentally determined or is based onpublished data known in the art and the offset value is a fixed constantwhich is added to compensate for breathing masks or other equipmentcomponents which may add deadspace to the circuit and therebyunacceptably skew the relationship between FRC and deadspace. The airwaydeadspace is the volume at which CO₂ crosses a selected threshold e.g.,(0.5 etCO₂ ). Dry gas is assumed in all equations.

Compensation is also made for parallel deadspace (See FIG. 10). Paralleldeadspace CO₂ concentration is calculated as a low-pass filtered versionof the mixed inspired CO₂ plus the airway deadspace times the previousend-tidal CO₂ concentration. The average CO_(2PDS) is etCO₂ times airwaydeadspace plus inspired CO₂ volume divided by the tidal volume.Breath-by-breath calculation of parallel deadspace, or unperfused space,concentration is therefore:PDS _(CO2)(n)={[FRC/(FRC+V _(t))]×PDS _(CO2)(n−1)}+({[ViCO₂+(deadspace×etCO₂(n−1))]/V _(t) }×[V _(t)/(V _(t) +FRC)]),where V_(t) is the tidal volume (the volume of the breath), PDS isparallel deadspace (i.e., space in the lung that is ventilated but notperfused by blood flow), etCO₂ is the concentration of CO₂ at the end ofthe exhaled breath, or “end-tidal,” “deadspace” is the volume in thetrachea and bronchi through which air must pass to get to the alveolibut in which no gas exchange occurs (also defined as “serial deadspace,”See FIG. 10) and (n−1) indicates the previous breath.

Alveolar CO₂ partial pressure (“P_(A)CO₂”) is calculated from theend-tidal CO₂ and the CO₂ in the parallel deadspace. Thus, ifetCO₂ =r×(P_(A)CO₂)+(1−r)PDS_(CO) ₂ ,then(P_(A)CO₂)=[etCO₂−(1−r)×_(PDS) _(CO) ₂ ]/r,where r is the perfusion ratio calculated as the ratio of perfusedalveolar ventilation divided by total alveolar ventilation, or(V_(A)−V_(PDS))/V_(A). The perfusion ratio r is estimated to be about0.92. Perfusion ratio can also be estimated by direct analysis ofarterial blood.

The (P_(A)CO₂) signal is then converted to CO₂ content using thefollowing equation:C_(CO) ₂ =(6.957×Hb+94.864)×1 n(1+0.1933(P_(CO) ₂ )),where CCO ₂ is the concentration of CO₂ and Hb is hemoglobinconcentration. In some instances, a hemoglobin count may be readilyavailable and is used in the equation. If hemoglobin (Hb) concentrationis not available, the value of 11.0 is used in the software program.

Baseline values of etCO₂ and VCO ₂, also referred to herein as “beforeCO₂ and before VCO ₂,” are those values which exist during normalbreathing and are calculated as the average of all samples between 27and 0 seconds before the start of re-breathing. Once a re-breathingepisode begins, the VCO ₂ value during re-breathing, also referred toherein as “during VCO ₂,” is calculated as the average VCO ₂ between 25and 30 seconds of re-breathing. The calculation of CCO ₂ during are-breathing episode is determined using a regression line to predictthe stable concentration of alveolar CO₂ (CCO ₂). To predict the CCO ₂at which the signal will be stable (i.e., unchanging), the CCO ₂ isplotted versus the breath-to-breath change in concentration. The line isregressed and the intersection between the CCO ₂ and zero ΔC_(CO) ₂ isthe predicted stable point.

Cardiac output is then calculated as follows:CO₂=[before VCO ₂−during VCO ₂]/[during CCO ₂−before CCO ₂].

The operation logic of the software program is briefly illustrated inthe flow chart of FIG. 11. The computer is programmed to detect the endof an exhalation 200, at which point the computer collects data from theCO₂ sensor and the flow meter and calculates CO₂, VCO ₂, inspired CO₂and airway deadspace values 202. The program then calculates FRC, at204, according to the equation previously noted. The program alsocorrects the VCO ₂ value, at 206, in accordance with the equationpreviously described. At thirty second intervals (thirty seconds onlybeing an average time, which may be adjusted higher or lowercommensurate with the size of the patient), at 208, the CO₂ and VCO ₂values are recalculated, at 210, to provide an average of those valuesbased on time, not on the variable time at which exhalation may end.

The program then calculates the estimated pCO₂ in the parallel deadspace212 and calculates the estimated pCO₂ in the alveoli 214 using theequations previously described. At that point, a re-breathing episode isinitiated 216 and a deadspace is introduced. Again, the computercollects data from the CO₂ sensor and the flow monitor of the apparatusand from that data, the change in VCO ₂ and alveolar CO₂ induced by theintroduction of the deadspace is calculated 218. If the calculatedchange in VCO ₂ is less than twenty percent (20%) of the baseline VCO ₂or if the change in partial pressure of alveolar CO₂ is less than 3 mmHg 220, then the operator is notified to increase the partialre-breathing deadspace 222 by increasing the expandable volumetricdimension of the adjustable deadspace of the apparatus. Baseline valuesare cancelled 224, then recalculated, as suggested by arrow 226. If,however, the change in VCO ₂ during re-breathing is greater than 80% ofbaseline values 228, then the operator is notified to decrease theadjustable deadspace of the apparatus by decreasing the volumetricdimension of the adjustable deadspace 230. The baseline values arecancelled 232 and recalculated, as suggested by arrow 234. Notably, thecomputer may notify the operator to make the necessary changes in theadjustable deadspace or, in an alternative embodiment, the computer maysignal mechanical means connected to the adjustable deadspace toincrease or decrease the volumetric dimension of the deadspaceautomatically.

Upon proper adjustment of the adjustable deadspace and the recalculationof baseline CO₂, VCO ₂, inspired CO₂ and airway deadspace values, thealveolar partial pressure (etCO₂) is converted by the software programto CO₂ content and the change in CO₂ content induced by the introductionof deadspace in the re-breathing episode is calculated 236. From thosevalues, cardiac output is calculated 238 in accordance with the equationpreviously described.

All references to times of data collection assume a thirty (30) secondre-breathing period. However, the actual length of time for periods ofre-breathing is dependent upon the patient's size, lung capacity andcardiac output determined from previous breathing cycles. The programalso controls the operation of the shunt valve or valves in there-breathing apparatus. The valve or valves are opened based on a timervalue determined by patient size, capacity and/or cardiac output.

The ventilation apparatus of the present invention provides a new andmore accurate means of determining cardiac output in patients. Thestructure and electronic capabilities of the present invention may bemodified, however, to meet the demands of the particular application.Hence, reference herein to specific details of the illustratedembodiments is by way of example and not by way of limitation. It willbe apparent to those skilled in the art that many additions, deletionsand modifications to the illustrated embodiments of the invention may bemade without departing from the spirit and scope of the invention asdefined by the following claims.

1. Apparatus configured to facilitate rebreathing by a subject,comprising: an airway conduit configured to communication with arespiratory system of a patient; a volume-adjustable deadspace incommunication with the airway conduit, the volume-adjustable deadspaceconfigured to be adjusted to one of more than two fixed volumes fromwhich the subject may rebreathe.
 2. The apparatus of claim 1, whereinthe airway conduit comprises at least a portion of the volume-adjustabledeadspace.
 3. The apparatus of claim 1, further comprising: aninspiratory conduit in communication with the airway conduit.
 4. Theapparatus of claim 3, wherein the inspiratory conduit comprises at leasta portion of the volume-adjustable deadspace.
 5. The apparatus of claim1, further comprising: an expiratory conduit in communication with theairway conduit.
 6. The apparatus of claim 5, wherein the expiratoryconduit comprises at least a portion of the volume-adjustable deadspace.7. The apparatus of claim 5, further comprising: an inspiratory conduitin communication with the airway conduit.
 8. The apparatus of claim 7,wherein the volume-adjustable deadspace comprises a plurality of shuntspositioned in communication between the expiratory conduit and theinspiratory conduit.
 9. The apparatus of claim 8, wherein a selectivelyactuatable valve is associated with each shunt of the plurality ofshunts.
 10. The apparatus of claim 7, wherein the volume-adjustabledeadspace comprises: at least one shunt positioned in communicationbetween the expiratory conduit and the inspiratory conduit; and at leastone volume-expandable section along at least one of the airway conduit,the expiratory conduit, and the inspiratory conduit.
 11. The apparatusof claim 10, wherein the at least one shunt includes at least oneselectively actuatable valve.
 12. The apparatus of claim 1, wherein thevolume-adjustable deadspace comprises at least one volume-expandableelement.
 13. The apparatus of claim 12, wherein the at least onevolume-expandable element communicates with at least the airway conduit.14. The apparatus of claim 13, further comprising: at least one valvelocated to facilitate selective direction of gas into the at least onevolume-expandable element.
 15. The apparatus of claim 13, furthercomprising: a flow restrictor located to, in conjunction with the atleast one valve, facilitate selective direction of gas into the at leastone volume-expandable element.
 16. The apparatus of claim 7, furthercomprising: a first volume-expandable element between the airway conduitand the inspiratory conduit; and a second volume-expandable elementbetween the airway conduit and the expiratory conduit.
 17. The apparatusof claim 7, further comprising: a volume-adjustable element positionablebetween the airway conduit and a source of ventilation gas.
 18. Theapparatus of claim 1, wherein the volume-adjustable deadspace comprisesan evacuation line for collecting exhaled gas and a gas-release elementassociated with the evacuation line for tailoring an amount or carbondioxide concentration of the exhaled gas within the evacuation line. 19.The apparatus of claim 1, wherein a volume of the volume-adjustabledeadspace is defined by a gas insufflation device and wherein thevolume-adjustable deadspace comprises a portion of an airway of thesubject and a portion of the airway conduit.